1. Technical Field
The present invention relates to a system for cooling an internal combustion engine having a turbocharger attached thereto and more particularly, to a cooling system for the turbocharger when the engine stops.
2. Description of the Related Art
A turbocharger for an internal combustion engine includes a turbine. As hot exhaust gas flows in the turbine, bearing lubricant is liable to be thermally degraded. To prevent a turbine rotor from being immovable under thermal influence due to insufficient lubrication, there has been made a proposal that a part of engine coolant is delivered to a water jacket for the turbocharger so as to cool bearings for the turbine rotor.
When the engine stops and thereby rotation of a pump for recirculating coolant is interrupted, coolant fails to be delivered to the water jacket for the turbocharger. This causes bearing portions for the turbocharger to be quickly heated up to an extremely high temperature. Consequently, it is unavoidable that bearing lubricant is thermally degraded due to the high temperature at the bearing portions in spite of the fact that lubricant remains at the bearings after the engine stops.
To obviate the foregoing problem, two prior inventions are disclosed, one of them being Japanese Laid-Open Patent NO. 219,419/1985 (hereinafter referred to as a first-mentioned prior invention) and the other one being Japanese Laid-Open Patent NO. 204,923/1985 (hereinafter referred to as a last-mentioned prior invention).
According to the first mentioned prior invention, a water jacket for the turbocharger is installed at a position substantially corresponding to the cooling portion of an engine cylinder head or higher than the portion, a coolant tank is installed at a position higher than that of the water jacket for the turbocharger and a coolant discharging passage extending from the water jacket is communicated with the interior of the coolant tank.
Steam generated in the water jacket for the turbocharger when the engine stops is brought in the coolant tank via the coolant discharging passage so that coolant in the coolant tank is fed to the water jacket for the turbocharger under the effect of respiration or it is fed to the water jacket for the turbocharger from a coolant passage for the engine via a coolant intake passage to cool bearing portions for the turbocharger.
On the other hand, according to the last-mentioned prior invention, when the engine stops, a solenoid valve in a delivery conduit is opened and hot coolant in the water jacket for the exhaust gas turbine in the turbocharger is transferred to a compensating tank (coolant tank) installed at a higher position than that of the turbocharger via the delivery conduit so as to cool it. After it is cooled, it is returned to an intake pipe leading to the water jacket for the exhaust gas turbine in the turbocharger. This prior invention discloses a thermo-siphoning that the exhaust gas turbine in the turbocharger is cooled by utilizing a so-called thermo-siphoning.
With either of the both prior inventions, steam generated in the water jacket for the turbocharger is transferred to an air chamber through the interior of the tank.
Consequently, with the first-mentioned prior invention, a part of steam containing air therein is induced into a radiator along with coolant in the tank.
With the last-mentioned prior invention, the steam which has been transferred to the air chamber in the coolant tank is induced into a heat exchanger along with air.
In this manner, steam containing air is unavoidably induced into the radiator according to either of the prior inventions. This leads to a problem that air lock occurs in a coolant pipe in the engine when the engine starts.